Different setup for selenium grid: using a selenium hub brings no complexity to the developers or to the already written tests; setting up differente remote controls for differente environments (IE on Windows, Firefox on Linux, Safari on Mac) only depends on having the Selenium hub

TestNG will allow to run the same tests in the different environments that were setup by Selenium Grid

Test Servers cost money: using the cloud allows to use on-demand servers, pay-per-hour, unlimited number of machines and start in a couple of minutes - no OS X offered by amazon

Create Amazon Machine Images for each different environment you need to test your application - The machines are killed after the tests are run and you only pay for the time you use those machines

Use Maven EC2 plugin to start amazon AMIs, stop at the end of test and pass user data